This sounds like rumination / intrusive thoughts and very much something a good therapist could help with. You also might just be depressed or anxious generally

This sounds like rumination / intrusive thoughts and very much something a good therapist could help with. You also might just be depressed or anxious generally, and if you get those more under control the death stuff might fade in to the background on its own. Death is scary. Therapy can't solve that. It can probably help you keep the fear of it from infecting the rest of your life. In the interim, check out Stoicism? The Stoics had a very hardheaded, practical sort of approach to philosophy of life, death, and suffering.